1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to equipment for coherent detection of optical quadrature-amplitude modulation (QAM) signals.
2. Description of the Related Art
Delivery of multimedia services (e.g., telephony, digital video, and data) that is implemented using optical phase-shift keying (PSK) signals or optical M-ary quadrature-amplitude modulation (M-QAM) signals has certain advantages, e.g., over that implemented using conventional electrical analog or digital signals. More specifically, some of the advantages are: the ability to carry various/multiple multimedia services over the same optical communication channel; the ability to maintain a selected bit-error rate (BER) with relatively low carrier-to-noise ratios; relatively high tolerance to nonlinear signal distortions; and relatively high spectral efficiency and transmission capacity. As a result, cable companies are upgrading their hybrid fiber coaxial (HFC) networks to improve/create a fully interactive, bidirectional optical network that can carry optical multimedia signals into and out of homes. It is projected that, in the near future, high definition television (HDTV) signals are likely to be delivered substantially exclusively over optical communication channels.
A typical coherent optical QAM receiver detects the received optical communication signal by mixing it with a local oscillator (LO) signal and processing the mixing results to determine the phase and amplitude of the communication signal in each time slot, thereby recovering the encoded data. To enable this phase and amplitude determination, the LO signal is typically phase-locked to the carrier wavelength of the communication signal using an optical phase-lock loop (PLL). More specifically, the PLL is configured to track the frequency and phase of the carrier wavelength and provide a feedback signal to the LO source, based on which feedback signal the LO source achieves and maintains the phase-lock.
Unfortunately, suitable coherent optical receivers are typically relatively difficult to design and/or relatively expensive to build. For example, a conventional, relatively inexpensive laser source might produce an optical signal that has a relatively large linewidth. If that laser source is used in a coherent optical receiver as an LO source, then its relatively large linewidth might produce a phase uncertainty/noise that can make the optical phase-lock between the LO and communication signals difficult to achieve and/or maintain. As a result, coherent optical receivers are often designed to have specially constructed laser sources and/or relatively complex optical PLLs, both of which can drive up the receiver cost by a substantial amount.